FIG. 1 is a block diagram of a transmitter-receiver using a conventional pulse generator. Here, a description will be given by taking a pulse generator used in an Ultra Wide Band (UWB) pulse system as an example.
A short pulse generator 10 generates a pulse having a narrow width from a baseband signal. The generated pulse is limited to a desired band by a band pass filter 11. After passing through the band pass filter 11, the pulse is amplified by an amplifier 12, so as to be made to pass through a switch 13, and is then transmitted from an antenna 14. Further, a signal received by the antenna 14 is made to pass through the switch 13, so as to be amplified by an amplifier 15. The amplified signal is made to pass through a band pass filter 16, and is then processed as a baseband signal.
FIGS. 2A and 2B are figures for explaining a conventional pulse generator.
FIG. 2A is a figure obtained by taking out only the pulse generator from the transmitter-receiver shown in FIG. 1. A single pulse is generated by the short pulse generator 10, and is then subjected to band limiting processing by the band pass filter 11, so that a single UWB pulse is generated. The band pass filter 11 cuts off a spectral portion having a center frequency of fc and having a band from f0 to f1, from the power spectrum of the pulse as shown in FIG. 2B. The UWB pulse is generated as a wave packet having a waveform shown on the right side in FIG. 2A. An amplitude of the waveform depends on an amplitude of the pulse outputted from the short pulse generator 10. The time interval between adjacent oscillation peaks in the wave packet is represented by τ equal to a reciprocal of the center frequency of the pass band of the band pass filter. That is, the wave packet oscillates at substantially the same frequency as the center frequency of the band pass filter.
The pulse generator includes the short pulse generator 10 and the band pass filter 11 (BPF) for allowing a signal of a desired UWB band to pass therethrough. The pulse ideally means a pulse having a pulse width infinitely close to zero and having a frequency spectrum extended, as shown in FIG. 2B, from a direct current to an ultra high frequency (to infinite frequency if the time width is zero). A UWB pulse having a desired frequency band can be obtained by inputting the pulse into a BPF designed so as to satisfy the UWB band spectrum mask. At this time, the UWB pulse is formed into a wave packet that oscillates at the center frequency fc of the BPF (the waveform of which is shown on the right side of FIG. 2A). An amplitude of the UWB pulse depends on a passage loss of the BPF, and the amplitude of the pulse outputted from the short pulse generator.
As shown in FIGS. 2A and 2B, a single UWB pulse is generated from a single pulse in the conventional technique. Thus, in order to amplify the amplitude of the single UWB pulse, it is necessary to increase the amplitude of the pulse itself. There are various methods for generating the pulse. However, for example, in the case where digital circuits (EXOR and AND circuits) are used, the preferred way is to increase the voltage of power source in order to increase the output amplitude of the pulse without deteriorating the pulse frequency characteristic (band). If a pulse having a doubled amplitude is to be outputted, it is simply necessary to double the power source voltage. This results in an increase in power consumption.
As a conventional technique, there is described in Japanese Patent Laid-Open No. 2004-146974 a method in which a synthesized waveform is generated by using a signal and its delay signal. There is also described in Japanese Patent Laid-Open No. 2002-335188 a technique in which a pulse inputted into an input of a band pass filter is made to pass through the band pass filter, so as to be formed into a wave packet.
In the conventional techniques, the pulse is made to pass through the filter, and hence the amplitude of the UWB pulse is reduced. Further, in order to obtain sufficient output power, it is necessary to increase the output amplitude of the pulse or to amplify the pulse by using an output amplifier. However, this results in an increase in power consumption.